Two identical designed devices on an integrated circuit have random differences in their behavior and show a certain level of random mismatch in the parameters which model their behavior. This mismatch is due to the stochastic nature of physical processes that are used to fabricate the device.
The mismatch of two CMOS identical transistors is characterized by the random variation of the difference including their threshold voltage Vt. Threshold voltage generally means the lowest gate voltage required to permit current to pass from a source to a drain in an insulated gate field effect transistor (FET). Threshold voltage is thus the level of voltage required to turn the transistor on. Threshold voltage is important because if it is too low, it is possible to suffer off state leakage.
Off state leakage is leakage current that occurs when the transistor is turned off. If zero volts are placed on the gate of the transistor, nominally, the transistor should be off and zero current should flow. If the gate is at zero volts, there is no inversion region and there should be no current. The source and drain should be like two back-to-back diodes. There may be a small amount of diode leakage, of course, particularly where the source and the drain are close together. There may be a certain amount depletion region extending from the source and the drain towards each other, because the critical dimension of the transistor, i.e., the gate length is reduced. If these depletion regions overlap, they may create source to drain leakage. The merging of these depletion regions may be defeated by an implant strengthening the depletion region beneath the gate. One side effect of such an implant is to raise the voltage required to turn the gate on, or raise the threshold voltage of the transistor.
One of the ways to dope the region under the gate between the source and drain to defeat off state leakage is called a halo implant. The halo implant, as is well known in the art, is an implant that is typically directed at the surface of the integrated circuit at an angle a little off the normal. One reason for implanting the halo implant after formation of the transistor is that the critical dimension or gate length of the transistor operates as a control in the variables required to establish threshold voltage. The critical dimension or gate length of the FETs on an analog circuit may vary. However, even a slight variation can affect the threshold voltage boost required by the implant. A longer critical dimension or gate length transistor will need a lower threshold voltage boost because the gate itself is separating the source from the drain leaving a large region between the source and drain which reduces the probability of source drain overlap. A shorter critical dimension or gate length transistor will need a higher threshold voltage boost because the gate itself is separating the source from the drain leaving a short region between the source and drain which increases the probability of source drain overlap.